Method and Apparatus for Gamma Ray Detection

ABSTRACT

A high sensitivity, three-dimensional gamma ray detection and imaging system is provided. The system uses the Compton double scatter technique with recoil electron tracking The system preferably includes two detector subassemblies; a silicon microstrip hodoscope and a calorimeter. In this system the incoming photon Compton scatters in the hodoscope. The second scatter layer is the calorimeter where the scattered gamma ray is totally absorbed. The recoil electron in the hodoscope is tracked through several detector planes until it stops. The x and y position signals from the first two planes of the electron track determine the direction of the recoil electron while the energy loss from all planes determines the energy of the recoil electron.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of copending U.S. patent applicationSer. No. 12/049,161, which is a divisional application of previouslyallowed U.S. patent application Ser. No. 11/349,115, filed Feb. 8, 2006,which is now U.S. Pat. No. 7,345,284, which is a continuation of U.S.patent application Ser. No. 11/102,825 filed on Apr. 11, 2005, which isnow U.S. Pat. No. 7,022,995, which is a divisional of U.S. patentapplication Ser. No. 10/434,075, filed May 9, 2003, now U.S. Pat. No.6,906,559, which is a continuation of U.S. patent application Ser. No.10/222,817, filed Aug. 19, 2002 now abandoned, which is a continuationof U.S. patent application Ser. No. 09/119,144, filed Jul. 20, 1998, nowU.S. Pat. No. 6,448,560, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/784,176, filed Jan. 15, 1997, now U.S. Pat. No.5,821,541, which claimed priority to U.S. provisional application No.60/011,135, filed Feb. 2, 1996. These and all other references areincorporated herein by reference in their entirety.

GOVERNMENT RIGHTS NOTICE

This invention was made with U.S. Government support under ContractNumbers R4MH49923 and DAMD17-96-1-6256 awarded by the Department ofHealth and Human Services and the Department of Defense, respectively.The U.S. Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to detection systems, and moreparticularly, to a method and apparatus for imaging gamma rays.

BACKGROUND OF THE INVENTION

The various organs and tissues of the human body fall prey to a myriadof different afflictions. For example, each year in the United Statesalone, approximately 180,000 women are diagnosed with breast cancer and46,000 women die of this disease. In all, 10 to 11 percent of all womencan expect to be affected by breast cancer at some time during theirlives. The causes of most breast cancers are not yet understood.Screening and early diagnosis are currently the most effective ways toreduce mortality from this disease.

Currently mammography is the most effective means of detectingnon-palpable breast cancer. However, mammography cannot determinewhether a lesion is benign or cancerous, typically one or more biopsiesmust be performed per lesion. Unfortunately the biopsy operation itselfis a very traumatic and costly operation that often results in somedegree of disfigurement. Therefore it is important to improve thespecificity of mammography thereby reducing errors, patient trauma, anddisfiguration from unnecessary biopsies. It is also important to reducehealth care costs by decreasing the number of unnecessary biopsies. Forexample, to detect 100,000 non-palpable cancers, approximately 500,000biopsies must be performed at a cost of about $5,000 per biopsy,yielding a total cost of approximately 2.5 billion dollars. Therefore, areduction of 50 percent would save about 1.25 billion dollars per year.

Palpable mass abnormalities of the breast are often difficult toevaluate mammographically. This is especially true for patients withdense or dysplastic breasts (approximately 35 percent of women over 50and 70 percent of women under 50) or those patients that exhibit signsof a fibrocystic change, for example due to radiation therapy. Forexample, invasive lobular carcinoma in dense breasts can attain a sizeof several centimeters and yet still show no mammographically detectablesigns. Furthermore, about 50 percent of all preinvasive cancers do notshow mammographically significant calcifications, thus decreasing thechances of detecting the malignant tumors.

Lastly, due to the interpretational limitations of mammography many highrisk patients (i.e., patients with a family history of breast cancer,patients with prior histologic evidence of cellular atypia, patientswith a prior history of breast cancer who have undergone lumpectomy andradiation therapy) may be forced to rely on random, tissue biopsiesperformed on suspicious areas. Unfortunately this technique typically,results in a high nonmalignant-to-malignant biopsy ratio.

A relatively new scientific tool that has allowed scientists andphysicians to address problems in physiology and biochemistry in thehuman body with low risk is emission computed tomography (ECT). ECTsystems are mainly used for the detection and imaging of the radiationproduced by radiotracers and radiopharmaceuticals. For example, byadministering biologically active radiopharmaceuticals into a patient itis possible to image organ functions in real time.

The two major instruments presently used for ECT are Single PhotonEmission Computed Tomography (SPECT) and Positron Emission Tomography(PET). These instruments have been used to study a variety of differentorgans and conditions including cerebral glucose consumption, proteinsynthesis evaluation, cerebral blood flow and receptor distributionimaging, oxygen utilization, stroke, heart, lung, epilepsy, breastcancer, dementia, oncology, pharmacokinetics, psychiatric disorders, andradio labeled antibody and cardiac studies. Since the SPECT and PETinstruments use different types of radiotracers, the metabolicactivities imaged are mostly different leading these two instruments tocomplement rather than compete with each other. The SPECT detectors haveproven especially useful for heart and brain imaging.

SPECT dates back from the early 1960s, when the first transverse sectiontomographs were presented by Kuhl and Edwards (1963) using a rectilinearscanner and analog back-projection methods. With the availability ofcomputer systems and the impetus of computer-assisted tomography usingtransmitted x-rays, nuclear medicine instruments were modified, and anumber of mathematical approaches to tomographic reconstruction weredeveloped in the early 1970s. Rotating Anger cameras and advances incomputers opened the way to three-dimensional SPECT systems. Recentlyinterest in SPECT increased as mathematical reconstruction techniquesimproved. They allowed for attenuation compensation, scattered radiationcorrection and the availability of new radiopharmaceuticals with higheruptake in the brain or other organs. The major limiting factors for theSPECT systems presently are the sensitivities (≈10 Cts s⁻¹ μCi⁻¹ pointand ≈21,000 Cts s⁻¹ cm⁻¹ volume), resolution (7 to 12 mm FWHM), size,and cost.

Present SPECT systems mainly use the rotating Anger camera. Manydifferent variations of the Anger camera and other smaller size rotatingsingle or dual instruments have been designed and used. Most of thecommercial instruments use NaI(Tl), CsI(Tl), CsF, BaF₂, BGO and otherrelated crystal detectors. The majority of the commercial instrumentsuse the Anger cameras made of NaI(Tl) crystals. All commercial SPECTinstruments use collimators for determination of the direction of theincident gamma rays. The main types are parallel and convergingcollimators. The converging fan or cone beam collimators produce highersensitivity but increase the complexity of the data analysis. Pinholeand slit collimators are also used. The collimators for high resolutionsystems eliminate about 99.9 percent of the incident gamma rays. Atypical collimator hole has an area of about 1 square millimeter and alength of 1.9 centimeters. Increasing collimator resolution decreasessensitivity and vice versa. Collimators made of high atomic numbermaterials such as lead which also produce considerable amounts ofscattered gamma rays on the inside surface of the collimator, therebyincreasing the scattered photon background.

Anger cameras are normally rotated on a gantry around the patient forabout 20 minutes to acquire sufficient data for a reasonable image. Thespatial resolutions are limited to about 7 to 12 millimeters althoughspatial resolutions are expected to reach 6 millimeters in the nearfuture. The best energy resolution at gamma ray energies is about 10percent, limiting the ability of Anger cameras to discriminate scatteredphoton background. Commercially available SPECT systems include ADACARC, GE Starcam, Elscint APEX, Trionix Triad, Digital ScintigraphicsASPECT and University of Michigan SPRINT II.

From the foregoing it is apparent that an improved gamma ray imagingsystem is desired.

SUMMARY OF THE INVENTION

The present invention provides a high sensitivity, high spatialresolution, and electronically collimated single photon emissioncomputed tomography (SPECT) system. Its primary sensitivity is in therange of 81 keV to 511 keV although it can be used to detect higherenergies of up to a few MeV by increasing the detector thickness forboth the hodoscope and the calorimeter. Both the direction and energy ofthe incident gamma ray photons is measured with high resolution. Themethod of determination of the photon direction eliminates the need fora mechanical collimator and the energy measurement discriminates againstthe scattered photon background.

The disclosed system is constructed from position sensitive, doublesided silicon strips with a strip pitch of approximately 1 millimeter orsilicon microstrips with a strip pitch much less than a millimeter.Preferably the system uses the silicon strip detectors. These detectors,varying in thickness from 150 micrometers to 2 millimeters, can producethe x and y coordinates of a photon interaction in a single wafer.

One embodiment of the system uses multiple planes of double sidedsilicon strip detectors with about 1 millimeter pixel size and athickness of 100 micrometers to 5 millimeters. The planes are separatedby a distance of between 0.2 and 2 centimeters, depending on the pixelsize and the required angular resolution. The smallest possibleseparation is always preferred to keep the depth of the detector smallwithout sacrificing spatial resolution. The incident gamma ray Comptonscatters in one of the detector planes, the dominant process for photonswith at least 50 keV energies in silicon strip detectors. The energy ofthe scattered electron in this detector plane is measured. The scatteredgamma ray with reduced energy can be absorbed in the calorimeter or inan another detector plane through the photoelectric effect or undergomultiple Compton scatters followed by a photoelectric effect. Theenergies of these subsequent interactions are also measured. If thescattered gamma ray photon is completely absorbed, the sum of the twoenergies gives the energy of the incident photon and the individualenergies and direction of the scattered photon give the scatter angle ofthe incident gamma ray. Thus the gamma rays emitted from a radionuclidecan be imaged without need for a collimator.

The scattered gamma ray photons can make a second Compton scatter andthen escape without further interaction. Also the photons alreadyscattered inside the patient will deposit lower total energy. Theseevents will produce a tail at lower energies in the energy spectrum.Such events can be discriminated effectively because the total energydetected is smaller than the known incident gamma ray energy. However, ahigh sensitivity mode may be applied with reduced angular resolution byadding the missing energy to the energy measured at the second scatter.This will dramatically increase the sensitivity but reduce angularresolution somewhat and will not allow the discrimination of thescattered photon background.

A calorimeter surrounding the silicon strip detector hodoscope absorbsthe Compton scattered photons. The calorimeter can be fabricated from aplane of silicon a few millimeters thick, CdZnTe strip and/or detectors,or CsI(Tl) crystals viewed by photodiode. The calorimeter can also beused as a second scatterer and/or a missing energy detector.

The double Compton scatter measurement determines the direction of theincident gamma ray to a cone with a half angle equal to the scatterangle. This type of measurement is new in nuclear medicine and requiresspecial data analysis software. The data analysis can be carried out bycone interaction, Maximum Likelihood or Maximum Entropy techniques.These are iterative techniques and require long computation times. A newdirect data analysis and imaging technique, Direct Linear AlgebraicDeconvolution (DLAD) method, can also be applied for real time imaging.

In use, the present system utilizes the higher uptake of certainradiopharmaceuticals by the organ or tissue of interest, therebyallowing the selected organ/tissue to be imaged. For example, malignanttissues preferentially absorb Tc-99m SestaMIBI and Tl-201 chloride ascompared to benign masses (except for some highly cellular adenomas).Therefore, these radiopharmaceuticals can be used to help diagnose anddifferentiate tumors from benign growths, for example in ascintimammography system for breast cancer detection and diagnosis.Possible mechanisms for uptake of Tl-201 chloride into tumor cellsinclude the action of the ATPase sodium-potassium transport system inthe cell membrane which creates an intracellular concentration ofpotassium greater than the concentration in the extracellular space.Thallium may be significantly influenced by this system in tumors. Inaddition, a co-transport system has been identified which also is feltto be important in uptake of thallium by tumor cells.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the gamma ray linear attenuationcoefficients for a silicon detector;

FIG. 2 is an illustration of the Compton scatter technique for detectinggamma rays;

FIG. 3 is an illustration of a typical double sided silicon microstripor strip detector;

FIG. 4 is the schematic cross section of the detector illustrated inFIG. 3;

FIG. 5 is an illustration of a side view of an embodiment of theinvention;

FIG. 6 is an illustration of a top view of the system illustrated inFIG. 5;

FIG. 7 is an illustration of the FEE readout chips and the silicon stripdetector planes mounted on a printed circuit board;

FIG. 8 is a graph illustrating the energy spectrum of Americium-241using a CdTe detector;

FIG. 9 is a graph illustrating the energy spectrum of Cobalt-57 using aCdZnTe detector;

FIG. 10 is a flowchart outlining the Monte Carlo gamma ray history for amodeled system according to the invention;

FIG. 11 is an illustration of a single head system according to thepresent invention;

FIG. 12 is a schematic diagram of a possible multi-channel siliconmicrostrip detector readout chip with fast data readout and triggeroutput capability; and

FIG. 13 is a block diagram of a real time data acquisition system foruse with the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS Gamma Ray Detection

The most probable interaction mechanism for 0.05 to 10 MeV gamma rays insilicon is the Compton scatter process. Therefore, the detection ofgamma rays in this energy range must use Compton interaction to havemaximum sensitivity. The detector must also have excellent angular andenergy resolution and a wide field-of-view. The best detection techniquethat has all of these features is the Compton double scatter method.This technique incorporates Compton scattering, photoelectricabsorption, and pair production. The three gamma ray interactionmechanisms are briefly discussed below.

Although a number of possible interaction mechanisms are known for gammarays in matter, only three major types play an important role inradiation detection: photoelectric absorption, Compton scattering, andpair production. Of these, only the first two play a major role inemission imaging. All of these processes lead to the partial or completetransfer of the photon energy to electron energy. They result in suddenand abrupt changes in the photon history where the photon disappearsentirely or is scattered through a significant angle.

FIG. 1 is an illustration of the gamma ray linear attenuationcoefficients for silicon microstrip detectors through these threeprocesses. The photoelectric absorption dominates below about 50 keV forsilicon. Compton scattering becomes important at about 50 keV and itstays the dominant process up to about 10 MeV, where pair productiontakes over. In the range of 81 to 511 keV which includes the nuclearmedicine range, the important detection process for silicon is Comptonscattering. Compton scattered gamma ray photons with energies less than50 keV are readily absorbed due to the photoelectric effect.

In the photoelectric absorption process, a photon undergoes aninteraction with an absorber atom in which the photon completelydisappears. In its place, an energetic photoelectron is ejected by theatom from one of its bound shells. The interaction is with the atom as awhole and cannot take place with free electrons. The photoelectronappears with an energy, E_(e), given by

E _(e) =hυ−E _(b)

where hυ is the incident photon energy and E_(b) represents the bindingenergy of the photoelectron in its original shell. For gamma rayenergies, hυ, of more than a 100 keV, the photoelectron carries off mostof the original photon energy. For silicon microstrip detectors, thisprocess is only important for low energy gamma rays in the range of 0.5to 50 keV. Photoelectric absorption falls nearly exponentially with anincrease in energy. Since the incident photon is totally absorbed it isnot possible to determine the direction of the incident photon.Therefore collimators must be used to determine the direction of originof the photon.

Compton scattering takes place between the incident gamma ray and anelectron in the absorbing material. In Compton scattering, the incidentgamma ray is deflected through an angle θ with respect to its originaldirection as illustrated in FIG. 2. The photon transfers a portion ofits energy to the recoil electron that was initially at rest. Becauseall angles of scattering are possible, the energy transferred to theelectron can vary from zero to a large fraction of the gamma ray energy.This has been a problem in the detection of gamma rays at energiesdominated by the Compton scatter process, since the detected recoilelectron alone does not give sufficient information to uniquelydetermine the energy and direction of the incident photon. This has beensolved by the Compton double scatter technique described below andillustrated in FIG. 2.

The total incident gamma ray energy, E_(y), and Compton scatter angle,θ, for the double scatter process are given by:

E _(γ) =E _(e1) +E _(γ1)

and

Cos θ=1−mc ²(1/E _(γ1)−1/E ₆₅)

The incident gamma ray first scatters by the Compton process in one ofthe silicon strip detectors 201, losing recoil energy E_(e1). Thescattered photon continues on until it interacts with another siliconstrip detector or is absorbed by a calorimeter 203. If the secondinteraction is photoelectric absorption, the full energy of the scatterphoton is measured and the energy of the incident photon and the scatterangle are determined. This is the dominant process for the calorimeteras it is made of high Z material and photoelectric absorption increasesexponentially with a decrease in the scattered photon energy. Anotherpossibility is that the second interaction can be another Comptonscatter where the photon escapes with a small amount of the energy. Ifthe energy of the escaping photon is sufficiently low, the energydetermination is not significantly effected. If there are enough siliconplanes, the escaped photon makes further interactions in subsequentplanes and gets fully absorbed by the photoelectric effect. All of theenergy measured after the second scatter is just added to the energy ofthe second scatter, E_(e2), to correct for the missing energy. If notenough silicon planes are used, for example due to cost considerations,a calorimeter can be placed such that it surrounds the sides and thebottom of the silicon strip detector hodoscope. The surroundingcalorimeter is used as a second scatterer to measure the energy anddirection of the scattered photon or to catch the escaping photons andcorrect E_(e2) for accurate incident photon scatter angle determination.Since the calorimeter is a high Z and high density detector orscintillator, there is a high probability that the escaped low energyphoton will be fully absorbed. The events that do not add up to the fullenergy of the incident photon are rejected to reduce scattered photonbackground.

The incident gamma ray direction lies on a cone segment in thefield-of-view with a half angle θ. The cone axis is determined by theinteraction positions in the first and the second scatters. This isbecause the direction of the scattered electron in the top scintillatoris not measured. The Compton scattered electrons with energies in therange of 81 to 364 keV are fully stopped within 0.03 and 0.3 millimetersof the silicon strip detectors, respectively. Therefore silicon stripdetectors with a thickness of 0.3 to 2 millimeter are ideal for thepresent system.

Silicon Microstrip Detectors

In the preferred embodiment of the invention, silicon microstripdetectors are used as the first scatterer (i.e., hodoscope). Siliconmicrostrip detectors have large active areas, excellent energy andposition resolution, and fast readout. Three inch diameter wafers,typically 200 to 500 micrometers thick, with parallel readout strips ofgreater than 25 micrometers pitch on one side have been available forfew years. Pitch size can have any value from 25 micrometers to severalcentimeters.

On the average, 1 electron-hole pair is produced per 3.6 eV of depositedenergy. The energy deposited by an 80 keV recoil electron fully stoppedin silicon is about 22,000 electrons (and holes) which can be collectedin less than 10 nanoseconds. This leads to pulse rise times of less than10 nanoseconds. Spatial resolutions of less than 10 micrometers in onedimension are obtainable by exploiting charge division between adjacentstrips. Superimposed on the signal is Gaussian-distributed noise relatedto the detector strip and preamplifier input capacitances. This noise orequivalent noise charge (e.g., ENC) is typically about 1,000 electronsat room temperature for detector capacitances of about 20 pF. Thus largesignal-to-noise ratios, on the order of 22, are obtainable for 80 keVelectrons.

To date, silicon detectors have been primarily used in high energyphysics experiments to detect minimum ionizing high energy chargedparticles. The Compton converter in the present invention is differentin that the recoil electron loses its entire energy in a single detectorwafer of about 1 millimeter thickness instead of depositing only part ofits energy like the minimum ionizing particles. The energy and angularresolutions improve as the number of electron-hole pairs created in thesilicon increase. For a 300 keV recoil electron stopping in silicon,about 83,000 electrons (i.e., 278 e/keV) are produced with an inherentenergy resolution of 0.8 percent (i.e., FWHM/E₀=2.35/√N where N is thenumber of electron-hole pairs). For 141 keV electrons stopping insidethe silicon wafer, the theoretical energy resolution is calculated to beabout 1.2 percent with a stopping distance for the recoil electron ofabout 0.1 millimeters. The theoretical resolution can be approached ifthe input capacitance and the preamplifier noise can be kept low. Theinput capacitance can be decreased substantially by mounting the chipsnext to the strips or building them on the same silicon. In the presentinvention a low noise, 64 channel front end mixed signal applicationspecific integrated circuit (ASIC) readout chips is used.

The individual detector thicknesses can be increased in order todecrease the number of required planes. Silicon strip detectors with a 1millimeter thickness are readily available while detectors withthicknesses of 2 millimeters have been manufactured. The energyresolution of silicon strip detectors is a dramatic improvement overscintillators (e.g., BC-523: 17% at 0.5 MeV).

Double sided readout silicon microstrip detectors with orthogonal stripson opposite sides have been developed. FIGS. 3 and 4 show the basicfeatures of a double sided silicon microstrip or strip detector. Thedistinct advantage with this configuration is that both x and ycoordinates of a traversing particle are determined in a single detectorplane. For single sided detectors, the junction side of a standard p+ndiode is segmented into many strips. For double sided detectors, theohmic side of the n-type silicon wafer is also segmented with orthogonalstrips to provide simultaneous readout of the particle impact point.Position resolutions well below a square millimeter on both sides can beachieved. The preferred detector in the present invention uses 200 to300 micrometer thick, double sided, silicon microstrip detectors withabout a millimeter spaced strips orthogonal on the top and bottomsurfaces. Such detectors are now commercially available and fit wellwith the present design. The x and y positions of the first twointeraction points on the recoil electron track determine the electrondirection. A combination of all interactions is used to determine theenergy of the recoil electron as well as the scatter angle.

In one embodiment of the invention, the detector is 6.4 centimeters by6.4 centimeters, the detector being fabricated from a 4 inch wafer. Inanother embodiment, 10 centimeter by 10 centimeter detectors are used.Bridged detectors with overall lengths exceeding 25 centimeters can alsobe used with the present invention. Bridging allows one preamplifier tobe connected to a series of strips on adjacent detectors withsignificant savings in electronics.

A simple Monte Carlo calculation using Monte Carlo Neutron Photon (MCNP)software from Los Alamos National Laboratory was performed. The MCNPsoftware gives the probability for a 141 keV photon to Compton scatterin varying total silicon thicknesses. For example, about 50 percent ofthe 141 keV photons will Compton scatter in a silicon detector 2centimeters thick. If 2 millimeter thick silicon strip detectors areused, then 10 planes will be required. For lower energy photons, a lowertotal thickness is required.

Another important advantage of silicon microstrip detectors is that theydo not need high voltages or cooling to low temperatures. Roomtemperature functionality is important to produce small size, low cost,and low power detectors. They also have a strong potential for massproduction. However, a significant number of wafers are needed toachieve the conversion rates required for high sensitivity, Their smallthickness and ultrasonic wire bonding capability render them goodcandidates for compact printed circuit board mounting with dataacquisition ICs placed next to them. The readout ICs are preferablydesigned to give fast trigger outputs when events occur and output theaddress and the analog content of the channel that has the data.

Calorimeter

Preferably a calorimeter is placed around and at the bottom of thesilicon microstrip detectors in order to absorb the escaping Comptonscattered photons. A variety of different high density radiationdetectors can be used. Many of these detectors are relatively high cost(e.g., HPGe, BGO, CdW0₄ and CsF) and several require cooling to liquidnitrogen temperatures (e.g., HPGe).

Sodium Iodide is the most popular high density scintillator. It has alarge light yield and its response to electrons and gamma rays is closeto linear over most of the significant energy range. The NaI(Tl) crystalis fragile and hygroscopic and therefore must be handled carefully andhermetically sealed. It has long decay time and is not suitable for fasttiming applications.

Cesium Iodide is another alkali halide that has gained substantialpopularity as a scintillator material. It is commercially available witheither thallium or sodium as the activator material and hassignificantly different scintillation properties with thallium. CsI hasa larger gamma ray absorption coefficient per unit size and is lessbrittle than NaI. The two forms of CsI scintillators, CsI(Na) andCsI(Tl) are discussed separately below.

CsI(Na) has an emission spectrum similar to NaI(Tl). Its light yield isalso comparable. CsI(Na) is hygroscopic and must be hermetically sealed.Therefore, CsI(Na) is similar to NaI(Tl) and has the same draw backs.

CsI(Tl) is different than NaI(Tl) and has unique properties. It is alsoonly slightly hygroscopic. Energy resolution of 5 percent FWHM at 0.662MeV has been obtained with 2.5 centimeter diameter by 2.5 centimeterthick CsI(Tl) scintillation crystals coupled to large area (e.g., 2.5centimeter diameter) mercuric iodide photodetectors. This is about 50percent better than the NaI(Tl) detectors. The mercuric iodidephotodiodes are not yet available as commercial devices. Resolution of 6percent at 0.662 MeV has been obtained for considerably smaller CsI(Tl)crystals using avalanche photodiodes. Large area PIN diodes coupled to 1centimeter by 2 centimeter CsI(Tl) crystals give a 7 percent resolutionat 0.662 MeV. These crystals produce 35 percent more photons per MeVthan NaI(Tl) and their light spectrum is much better matched to thesensitivities of the photodiodes. A key to improved energy resolution isgood light collection by matching the areas of the crystals to those ofthe photodiodes.

An important property of CsI(Tl) is its variable decay time fordifferent particles. Therefore pulse shape discrimination techniques canbe used to differentiate among various types of radiation such aselectrons, protons and alpha particles. The CsI(Tl) light output isquoted lower than NaI(Tl) for bialkali photomultiplier tubes (PMTs)(FIG. 5). The scintillation yield is actually found to be larger thanthat of any other scintillator because its light emission peaks atlonger wavelengths. It can be used with photodiodes with extendedresponse in the red region of the spectrum. Its energy resolution isequal to or better than the energy resolution of the NaI(Tl) crystals.For these reasons CsI(Tl) crystals are used in at least one embodimentof the invention.

CdTe, CdZnTe, HPGe and HgI₂ are solid state detectors and can be made inarrays for position sensitive applications. They are high Z and highdensity crystals. They are used to detect x-rays and gamma rays directlywithout need for photomultiplier tubes or PIN and avalanche photodiodes.They produce much better energy resolution than the other detectorswhich require photomultiplier tubes or PIN and avalanche photodiodessince they convert the energy deposited by the x-ray and gamma rayphotons into light, not electron-hole pairs.

High purity germanium (HPGe) offers excellent high energy resolution andexhibits moderate gamma ray absorption properties, making it thedetector of choice for high accuracy spectroscopy. Unfortunately sinceit only works at liquid nitrogen temperatures, bulky refrigerationsystems are required which further increase the cost of this detector.HPGe is available in single small crystals and works by collecting theelectron hole pairs produced inside the crystal similar to the silicondetectors and does not require PMTs. Because of the large cost thisdetector is mainly used for applications which require ultra high energyresolution and small size detectors.

BGO, CdWO₄ and CsF are excellent high density and high Z scintillators.They have lower energy resolution and light output. Their maximum lightemissions peak around 430 nanometers, similar to NaI(Tl), and requirePMTs for detection. CdW0₄ and especially CsF have shorter decayconstants and faster rise times than the others and can be used fortiming. However, since the preferred detector of the present inventiondoes not use time-of-flight to determine the direction of the scatteredgamma ray photon, good time resolution is not important.

The preferred room temperature detector for the calorimeter of thepresent invention is CdTe or CdZnTe. These detectors are described inmore detail below.

System

The present invention, relying on isotope uptake in the region (i.e.,organ or tissue) of interest, can be used for a variety of differentapplications ranging from real-time monitoring (e.g., blood flow througha heart valve) to lesion diagnosis (e.g., breast lesions). The disclosedsystem is relatively compact while offering improved efficiency andspatial resolution. An obvious benefit of the improved efficiency of thepresent invention is a significant decrease in the observation time orthe radiopharmaceutical dosage.

One embodiment of the invention is illustrated in FIGS. 5 and 6. Anobject 501 to be imaged such as a breast, brain, or other organ ortissue is placed at the front of system 500. The hodoscope is made up ofbetween approximately 1 and 100, and preferably between approximately 10and 25, silicon strip detector planes 503. Detector planes 503preferably have a thickness of between 0.5 to 1 millimeter, the selectedthickness being dependent upon the desired performance as well as theavailability of the detectors. The total Compton scatter probabilitywill vary from approximately 50 percent for ten 2 millimeter thicksilicon strip detectors to approximately 35 percent for twenty five 0.5millimeter thick detectors. The active area of the silicon stripdetectors can be increased by mounting four or more detectors 601 sideby side on each plane 503 as shown in FIG. 6.

The hodoscope height depends strongly on the number of detector planes503 as well as the separation between the planes. In the illustratedembodiment, 1 millimeter thick detectors are used giving a planeseparation of about 1 centimeter and a hodoscope height of about 15centimeters. Preferably these values as well as the thickness andseparation of the silicon detectors is optimized through Monte Carlosimulations and experimental study.

The calorimeter is made from about 2 millimeter thick CdTe or CdZnTestrip or pad detectors 505. The reason for this selection is the higherenergy resolution obtained from CdTe/CdZnTe detectors especially atlower energies. A CsI(Tl) calorimeter can also be used.

In the illustrated embodiment, calorimeter 505 is a single layer placedaround, and as close as possible, to the hodoscope. The proximity of thecalorimeter to the hodoscope is limited in order to avoid introducingsignificant angular resolution degradation due to the geometriccombination of pixels. The gap at the bottom is due to the energythreshold of the silicon detectors which is typically greater than 5keV. The incident photons that deposit energy less than the thresholdenergy will not be detected in the hodoscope and such small anglescatters need not be stopped at the calorimeter. The geometry, strippitch, thickness, shielding, and the size of the gap at the bottom ofthe calorimeter is optimized by Monte Carlo simulations. The detectorgeometry is optimized to any form such as square, rectangular,cylindrical, spherical, parabolic, etc. that gives the best results fora specific application.

A shield 507, preferably made of a material such as lead or tungsten, isplaced in front of and around the calorimeter to reduce the background.Shield 507 is especially important for certain applications. Forexample, if imaging system 500 is used for the detection of malignantbreast tumors, a radiopharmaceutical such as Tc-99m SestaMIBI or Tl-201may be used. In either case, substantial amounts of theradiopharmaceutical may be taken in by the heart thus requiring adequateshielding to achieve the desired signal-to-noise ratio.

In an alternate embodiment used to obtain an intermediate improvement insensitivity, a slot collimator 509 is placed at the aperture of thehodoscope. Collimator 509 confines photons to the planes defined by theslots. Collimator 509 will therefore slice the event cone inherent in aCompton scatter detector into two sections, one section defining thetrue event direction and the other section defining the false eventdirection. The false event directions normally lie outside the viewedobject, especially for large scatter angles. Thus the correct andincorrect directions can be defined for each event and all false eventscan be rejected.

In the preferred embodiment of the invention, each plane of thehodoscope is made from four 1 millimeter thick silicon strip detectors601 with an active area of approximately 6.4 centimeters by 6.4centimeters each. Detectors 601 are mounted as close to each other aspossible. Therefore, in this embodiment the active area is approximately12.8 centimeters by 12.8 centimeters or about 164 square centimeters.The number of detector planes 503 is a function of the application. Forexample, if system 500 is to be used to image breast tumors, thehodoscope preferably has 10 detector planes with a 0.5 centimeterspacing. For other organ imaging applications the hodoscope has between15 and 25 detector planes with an approximately 1 centimeter spacing.

Silicon strip detectors 601 are mounted on a printed circuit board (PCB)603 or a ceramic holder as illustrated in FIGS. 6 and 7. Front endelectronics (FEE) readout chips are mounted on the PCB proximate to thesilicon strip detectors 601 either on the front surface of the PCB atlocations 701 or on the back surface of the PCB at locations 703. Afan-in from the strip pitch to the FEE chip pad pitch is done on thesilicon strip detector for reliability and ease of ultrasonic wirebonding.

Preferably silicon strip detectors 601 are designed and fabricated usingthe new FOXFET AC coupling technique on both the junction and ohmicsides. This technique improves the signal quality, especially at theohmic side, since the bias resistor formed through the FOXFET techniqueis much larger than with other techniques. It also eliminates externalcapacitances and resistors which become bulky, require large realestate, and are costly when large numbers of channels are used.Preferably high radiation resistant FOXFET silicon strip detectors areused which significantly increases the reliability of the system.

FOXFET silicon strip detectors are commercially available and showexcellent response to low energy (i.e., 81 to 5 11 keV) photons. Bylowering the dark current and reducing the junction thickness todecrease strip capacitance, a reduction in detector and electronic noiseshould be achieved, thereby improving energy and spatial resolution.

The small size of the active area and the dividers between the foursilicon strip detectors at each plane do not cause problems such as sidetruncation or image gaps. This is because the disclosed techniqueinherently has a large field of view and the detector active area can besmaller than the imaged organ of the patient. Smaller active area, deadstrips within a plane, or gaps in between the silicon strip detectors,only reduce the detection efficiency while not affecting the image. Thusas a result of the Compton scatter technique, image defects arevirtually eliminated due to the system's tolerance to defects,

Although the invention can be used without a calorimeter, the preferredembodiment includes a calorimeter utilizing CdZnTe strip detectors.These detectors have excellent energy resolution for 10 to 250 keV gammarays, or for 250 to 600 keV gamma rays if thick detectors are used.Therefore, CdZnTe is especially useful to work with ^(99m)Tc and ²⁰¹Tl,the most commonly used radionuclides.

The second choice for the calorimeter are CsI(Tl) crystals coupled tospecially developed PIN photodiodes. The energy resolution of thesecrystals, contrary to CdTe detectors, increases as the gamma ray energyincreases. Therefore, they are an excellent choice for source gamma rayswith energies greater than 250 keV.

At higher energies, the thickness of silicon required to stop the gammarays becomes larger, requiring multiple Compton scatters prior toabsorption. If a calorimeter is used, the incident photon only needs tomake a single Compton scatter in the silicon hodoscope.

The energy resolution for a 1 by 1 by 2 cubic centimeter crystal ofCsI(Tl) is approximately 5 percent at 662 keV using a ¹³⁷Cs source, thusshowing that a CsI(Tl) calorimeter can be used with the presentinvention. A CsI(Tl) calorimeter with smaller crystals can be used atlower energies without a significant impact on the stopping power of thecalorimeter. For example, a 0.5 centimeter long CsI(Tl) crystal canabsorb 95 percent of 141 keV photons.

In general the present invention has three basic embodiments dependingupon the intended use. The first embodiment is intended for relativelylow energy, i.e., between about 81 and about 250 keV. Due to the lowenergy, this embodiment can be fabricated either with or without acalorimeter. If a calorimeter is used, preferably it is a CdZnTecalorimeter. The second embodiment is intended for relatively highenergy, i.e., between about 250 and about 511 keV or greater. Thisembodiment uses both the hodoscope and the calorimeter, the calorimeterutilizing either CdZnTe or CdI(Tl). The third embodiment can be usedthroughout the entire energy range, albeit with slightly lowerefficiency and spatial resolution. In this embodiment preferably aCsI(Tl) or a thick (e.g., 0.5 to 1 centimeter) CdZnTe calorimeter isplaced behind a 2 millimeter thick CdZnTe plane. The CdZnTe calorimeteris useful for low energy radionuclides while both the CdZnTe and theCsI(Tl) calorimeter can be used with the silicon hodoscope for highenergy sources. In such an arrangement interactions in all threesections may happen and can be used as viable data for imaging.

The origin of CdZnTe is cadmium telluride (CdTe) detector. CdTe containsrelatively high atomic numbers (i.e., 48 and 52) with a large enoughbandgap energy (i.e., 1.47 eV) to permit room temperature operation.This bandgap limits resistivities to the low-109 ohm-centimeter range,resulting in relatively large room temperature dark currents. CdTe has adensity of 6.06 grams per cubic centimeter and the energy required tocreate a single electron-hole pair is 4.43 eV. The hole mobility isabout a factor of 30 slower than the electron mobility. The hole lifetimes are also very short due to the low mobility enhancing the effectsof trapping and recombination. Improvements in hole collectionefficiency can be obtained by using higher purity materials.

In CdTe, for typical gamma ray energies the probability of photoelectricabsorption per unit pathlength is approximately 100 times larger than insilicon. For example, CdTe is opaque to low energy x-rays forthicknesses in the range of a millimeter. However, the energy resolutionof CdTe is not comparable to silicon detectors for low energy x-rays dueto poor hole collection efficiency. The room temperature measured energyresolution for CdTe detectors is 3.5 keV at 122 keV.

Many problems associated with CdTe detectors are related to a specifictechnique of crystal growth referred to as the traveling heater method(THM). This technique requires that the crystals be doped with anelement such as chlorine in order to achieve high resistivity.Unfortunately, chlorine doping is generally associated with detectorlong-term reliability problems as well as various operatinginstabilities such as counting rate polarization. Lastly, due to the lowyield of detector grade material using this technique, detector pricesare relatively high.

The CdZnTe detectors were specifically developed as gamma ray detectorsby several companies. By using a high pressure Bridgman (HPB) techniqueto grow the crystals, improvements in both size (e.g., up to 10centimeter diameter crystals weighing over 10 kilograms) and yield(e.g., over 70 percent) have been realized. These crystals exhibituniform, near-intrinsic resistivity without doping. Detectors fabricatedfrom HPB grown crystals exhibit excellent stability, reliability andlifetime. Furthermore, the HPB process can be used to grow high qualitycrystals of Cd_(1-x)Zn_(x)Te throughout the entire alloy compositionrange. Alloying ZnTe with CdTe increases the bandgap, resulting in muchhigher resistivities and correspondingly lower leakage currents thanCdTe.

The energy resolution of both the CdTe and CdZnTe detectors for 10 to300 keV energies is important for the present invention. FIG. 8 showsthe energy spectrum of an Americium-241 source with a CdZnTe detector.The x-ray emissions at 13.9, 17.7, 20.8, 26.4, and 59.5 keV (with escapepeaks for characteristic K x-rays from Cd at 36.5 keV and Te at 32.5keV) are clearly seen with good energy resolution. The slightly lowerenergy tail observed for the 59.5 keV peak is typical of that observedwith CdZnTe detectors and is due to incomplete charge collection forsome of the events.

The energy spectrum of a Cobolt-57 obtained by a 2 millimeter thickCdZnTe crystal is shown in FIG. 9. The low energy tail is clearly seenat higher energies.

In one embodiment of the invention, CdZnTe strip detectors produced fromCd_(0.8)Zn_(0.2)Te wafers were used. The strip pitch was 1 millimeterwith a total of 32 strips on each side, providing an active area of 3.2centimeters by 3.2 centimeters. The strips on each side were orthogonalto each other in order to provide both the x and y dimensions for aninteraction. The CdZnTe strip detectors can be from 1.5 to 2.5millimeters thick. Two-dimensional arrays of CdZnTe pad detectors canalso be used. Pad detectors generally provide better results than stripdetectors since they do not have the positional ambiguity associatedwith strip detectors when there is more than one event simultaneouslyinteracting with multiple detectors.

A full size cylindrical system according to the present invention wasmodeled using the MCNP Monte Carlo code discussed above. The internaland external radii of this cylindrical system were 15 and 50centimeters, respectively. The length of the modeled system was 50centimeters long. The phantom used at the center was a standard cylinder20 centimeters in diameter 20 centimeters long filled with water and 1μCi/cc of a ^(99m)Tc radiotracer. Double-sided silicon strip detectorsthat were 1 millimeter thick with a 1 millimeter strip pitch weremodeled in cylindrical form. All together, 36 planes were placed insidethe detector with a 1 centimeter separation between planes. The totalthickness of the 36 planes is 3.6 centimeters corresponding to a 72percent Compton scatter probability. The 141 keV gamma rays, produceduniformly in all directions in the phantom, were tracked along theirpaths until they were fully absorbed or escaped through the back orsides of the detector. A 3 keV energy threshold of detection was imposedon each silicon detector. A calorimeter behind or at the sides of themodel was not used.

The history of the 141 keV photon was traced by Monte Carlocalculations, the results of which are shown in FIG. 10. The Monte Carlocalculations were carried out for about 100,000 events and the resultsscaled to the simulated phantom. The 141 keV photons scattered in thephantom are effectively discriminated by the high energy resolution.This significantly reduces the major scattered photon background. Thesingle scatter photons are rejected as their directions cannot bemeasured. The events which create multiple electrons in the samedetector wafer are also rejected. Most of these are probably due toknock on electrons by the recoil electron. In most cases the secondaryelectrons are created and absorbed within the pixel size at the positionof the interaction. These events are legitimate and can be used inimaging.

The forward and backscattered gamma ray events can be easily identifiedbecause of the strict relationship imposed by the Compton scatterformula. This is especially true at low photon energies. For 141 keV^(99m)Tc gamma rays, the energies deposited in the interaction pointnearest to the patient are limited to 0 to 31.5 keV and 110.5 to 90.9keV for forward and back scattered photons (i.e., θ<90°), respectively.For the interaction point farthest from the patient (i.e., 90°<θ<180°),the energies deposited are 31.5 to 50.1 keV and 141 to 110.5 keV for theforward and back scattered photons, respectively. Since the energy ateach interaction plane is measured separately, such widely differingenergy deposition for the forward and backward scattered photons iseasily identifiable and the direction cones can be calculated. Thereforethe backscattered events that deposit full energy in the detector aregood events and can be used in imaging.

The point sensitivity is estimated to be about 1,500 Cts s⁻¹ μCi⁻¹. Thevolume sensitivity of the simulated detector is about 500,000 Cts s⁻¹cm⁻¹ found by dividing the good event rate, 1×10⁷ cts s⁻¹, by the lengthof the phantom. The sensitivity of the invention strongly depends on theamount of silicon used and can be improved further by increasing thenumber of silicon detectors. The number of silicon strip detectors canalso be decreased to reduce cost since the sensitivity is high and somesacrifice is affordable.

The FWHM uncertainty in the cone half-angle, Δθ, due to a detector offinite energy resolution (FWHM), ΔE_(e1) and ΔE_(e2) at first and secondscattering planes can be calculated using the Compton scatter formula:

${\Delta\theta} = {\frac{{mc}^{2}}{E_{\gamma}^{2}{Sin}\; \theta}\left\{ {{\Delta \; E_{e\; 1}^{2}} + {\left\lbrack {\frac{E_{\gamma}^{2}}{E_{\gamma \; 1}^{2}} - 1} \right\rbrack^{2}\Delta \; E_{e\; 2}^{2}}} \right\}^{1/2}}$

where mc² is the electron rest energy (511 keV), θ is the Comptonscatter angle, and E_(γ) and E_(γ1) are the incident and scatteredphoton energies. Applying the formula, the energy resolution due to thestatistical fluctuation for electrons stopped inside the siliconmicrostrip detectors varies from 1.3 percent at 100 keV to 0.75 percentat 350 keV. The electronics noise of the detector is about 2 keV.Therefore the total energy resolution is dominated by the electronicsnoise which is the same for both the converter and the calorimeter.

The angular resolution is calculated with an energy resolution of 2 keV(FWHM) where Δθ for forward scattered gamma rays (i.e., θ<90°) variesfrom 5° at a θ of 30° to about 3.2° at a θ of 70° for 141 keV (^(99m)Tc)incident photons. The same calculation carried out for 364 keV ¹³¹lgamma rays gives angular resolutions of approximately 1° for a θ ofbetween 20° and 90°. Thus the angular resolution improves significantlywith an increase in the photon energy. Also the effects of amplifiernoise are reduced as more electron-hole pairs are created by higherenergy scattered electrons. At a distance of 20 centimeters theseangular resolutions produce effectively 6 to 3.5 millimeter spatialresolutions for 141 keV gamma rays and 3.5 to 1.5 millimeter spatialresolutions for 364 keV gamma rays. At a distance of 2.5 centimeters thesame energy gamma rays produce 2.2 to 1.4 millimeter spatial resolutionsand 0.4 millimeter spatial resolutions, respectively.

The geometric angular resolution, Δθ_(Geom), gives the axis of the imagecone and is dependent upon the silicon microstrip detector pixel sizeand the distance between the first two scatters. The FWHM value can becalculated similar to that for a collimator. Normally the geometricangular resolution is kept much smaller than the scatter angle variationwhich depends strongly on the energy resolution as shown above. It iseasier to adjust the geometric angular resolution in a siliconmicrostrip detector as the strip or pixel pitch dimensions can be assmall as 25 microns. The pixel size for the simulated model is 1 squaremillimeter.

The Monte Carlo analysis shows that about 1×10⁸ photons per second outof 2.3×10⁸ enter the detector as shown in FIG. 10. As noted above, thesimulated detector has 36 cylindrical planes with an average area ofapproximately 10⁴ square centimeters and about 75 percent of the photonsmaking an interaction (i.e., 7.5×10⁷ photons per second). Assuming eachsilicon microstrip detector wafer has dimensions of 5 centimeters by 5centimeters, the singles rate in each wafer is about 5,000 Cts/s. Suchsingles rates are not excessive for silicon microstrip detectors whichproduce about 20 nanosecond long pulses. The coincidence requirementfurther reduces the actual readout rate to about 670 per second.Therefore dead time per detector is not a problem. However, the totalcount rates of the whole detector will be high. This problem is solvedby establishing high level parallelism in readout electronics for whichthe silicon microstrip detectors are highly suitable. One possible wayis to divide the detector into many radial sections and read eachsection individually. If it is divided into 20 sections than readoutrate at each section will be about 500 kHz which can be easily handledby a standard CAMAC data acquisition system. The data rate will be evensmaller due to some loss of events at the edges when the photons scatterinto adjacent sections. This will also reduce sensitivity somewhatunless such events can be recovered by the electronics. There is also alarge number of channels to readout. This is solved by using highdensity ASIC chips directly connected to the microstrips. Chips whichproduce a trigger signal when there is valid data and connect the stripthat contains information to the output can be used.

FIG. 11 is an illustration of a simple, single head apparatus fabricatedaccording to the present invention and used for system testing. Thehodoscope is comprised of 10 layers 1101 of silicon strip detectors inwhich each detector layer 1101 has an area of 12.4 centimeters by 12.4centimeters with a thickness of 1 millimeter. The distance between eachlayer 1301 is 0.5 centimeters. A calorimeter 1105 is symmetricallypositioned 6 centimeters after the last hodoscope layer 1101.Calorimeter 1105 is a 2 millimeter thick CdTe detector with an area of50 centimeters by 50 centimeters. A 141 keV gamma ray source 1107 with a0.5 centimeter diameter is centrally positioned 10 centimeters above thefirst silicon plane 1101. A threshold energy of 10 keV was applied tothe silicon strip detectors. An event is generated only if the incidentgamma ray makes a Compton scatter in one of the silicon planes and alsointeracts at the calorimeter.

There were a total of 56,234 triggers out of 106 incident gamma rays.The low efficiency, about 5.6 percent, was due to the overall smallsilicon thickness of 1 cm (i.e., approximately 30 percent Comptonscatter probability). The low efficiency was also a result ofcalorimeter 1105 not covering the sides of the hodoscope since most ofthe photons scattered at an angle of greater than 70° would not bedetected. Lastly, since the thickness of calorimeter 1105 was only 2millimeters, the absorption probability was approximately 85 to 50percent for 90 to 13 1 keV scattered photons due to the Comptongeometry. About 0.56 percent of the incident photons produced aphotoelectric absorption inside the silicon hodoscope. The total numberof events that deposited full energy in the detector was 4.2 percent. Ifthe events are restricted to total absorption in calorimeter 1305 afterCompton scattering once in the silicon hodoscope, about 2.8 percent ofthe incident photons were detected. This excludes totally absorbedevents in silicon after 2 or more Compton scatters in the hodoscope.

Two more sources, 1109 and 1111, were added to the above single sourcediscussed above. Source 1109 was positioned 2 centimeters from thecenter source in the −x direction while source 1111 was positioned 1.5centimeters in the +x direction. All the sources produced 141 keV gammarays sprayed into a cone the size of the hodoscope. The photons producedby the sources at the sides missed part of the detector aperture due totheir position. Therefore, the strongest source imaged was the centrallyplaced one. The images were obtained using a standard analysis program.This program integrated the overlap of each event ring at thecorresponding pixel. The energies deposited at the hodoscope and thecalorimeter are randomly Gaussian distributed using the calculatedenergy resolutions for the preferred prototype system to simulateauthentic spatial resolution.

The present Compton double scatter detectors provide two basicparameters for each event related to the incident photon direction; thescattered photon direction and the Compton scatter angle. The DirectLinear Algebraic Deconvolution (DLAD) technique can be used to analyzethis information.

A concise explanation of the DLAD technique is provided below. Thereconstruction of the source image from the Compton double scatter datacan be represented by the following general formula:

D(χ, Ψ, φ, E)=∫_(χ,Ψ,E) I(χ₀, Ψ₀ , E′)R(χ, Ψ, χ₀, Ψ₀ , φ, E′, E)dχ ₀ dΨ₀ dE′+B(χ, Ψ, φ, E)

In the above formula, D(χ,Ψ, φ, E) is the actual Compton scatter dataobserved by the detector in appropriate coordinates; χ and Ψ are thecoordinates of the rectangular image plane; φ is the Compton scatterangle; E is the energy of the incident photon; I (χ_(o), Ψ_(o), E_(o))is the true image of the source and is not a function of the Comptonscatter angle; R(χ, Ψ, χ_(o), Ψ_(o), φ, E′, E) is the response functionof the detector; and B(χ, Ψ, φ, E) is the gamma ray background. Normallythe calculation is carried out for all energies within the detectorsensitivity to determine the total gamma ray flux and for certain energybands to obtain an energy spectrum. For application to the presentinvention, the energy spectrum is used to discriminate the scatteredphoton background. The calculation can also be done for differentscatter angle bands. D and I are normally referred to as the data andthe image spaces, respectively.

The response function in the DLAD technique is the concentric ringsobtained by mapping the scattered photon direction vector in the imageplane. This can be used as an ideal detector response function. The truedetector response function, R, can be represented by

R _(ij, Φ) _(s) =ε(E, θ _(j), Φ_(s))·ΔΦ_(s) ·PSF·G(θ_(i))

where i and j define the bins in the data and image spaces,respectively; Φ_(s) is the calculated Compton scatter angle as given byCompton scatter formula; ε is the detector efficiency; θ_(i) and θ_(j)are the incident zenith angles in data and image spaces, respectively;ΔΦ_(s) is the scatter angle interval; PSF is the point spread function;and G(θ_(i)) is the geometric factor. The PSF is the distribution of thescattered photon vectors in the image plane. The PSF can be representedby the two dimensional normal distribution

PSF=C(θ_(j)Φ_(s))e ^(−{[(Φ) ^(l) ^(−Φ) ^(s) ⁾ ² ^(])/[2σ ² ^((E)]})

where C is the normalization constant determined by the requirement thatPSF×G(θ_(i)) is equal to 1. The PSF and G(θ_(i)) are symmetric in theazimuth, thus giving a two-dimensional image. The present invention canproduce three-dimensional images due to the Compton scatter process.Therefore, either two-dimensional image slices parallel to the converterplanes are produced or a direct three-dimensional image can beconstructed.

The DLAD technique can produce fluctuations on the image space that aredue to the geometric factor forcing data space to zero at the cornersand edges of the field-of-view where the data may be scarce and thePoisson fluctuations are large. This effect can be improved by applyingthe positivity requirement. The positivity requirement is based on thefact that in image space one cannot get negative fluxes. The positivityconstraint has been introduced into DLAD. The new technique is calledConstrained Linear Algebraic Deconvolution (CLAD).

An important technological requirement for the present invention is amultichannel front end electronics (FEE) chip with self trigger outputto readout the silicon strip and calorimeter detectors. A detaileddescription of a FEE chip is provided in U.S. Pat. No. 5,696,458, issuedDec. 9, 1997 and in co-pending U.S. patent application Ser. No.08/866,117, filed Jun. 27, 1997, now U.S. Pat. No. 6,150,849, issuedNov. 21, 2000, both disclosures of which are incorporated herein for allpurposes.

The preferred FEE chip is a 64 channel, charge sensitive, mixed signalASIC CMOS chip, a version of which is illustrated in FIG. 12. Eachchannel of the chip consists of an analog section and a digital section.The input from the silicon strip detector is directly coupled to a lownoise, charge sensitive amplifier. The outputs of the charge sensitiveamplifier are connected to a shaper amplifier with a time constant ofabout 100 to 200 nanoseconds. The output of the shaper amplifier goesinto the track and hold (T/H) switch. The T/H switch can be controlledexternally or activated internally from the trigger output with a delayset to turn on the hold at the peak of the shaped pulse. The T/H switchis connected to the input of the buffer amplifier through the voltagefollowing capacitor. When the T/H switch is open the voltage on thecapacitor is held constant and the voltage level is buffered on to theanalog output switch. A shift register connects each buffer output tothe single analog output pin in sequence, from input 1 to N, by anexternal clock input. The shift register also has an external clearinput to reset it and a clock output to daisy chain it to other readoutchips. Only one clock input is sufficient if the clock outputs areconnected in serial to the clock inputs of the adjacent readout chips.The charge sensitive amplifier outputs can be fanned out to comparatorswith a common external level adjustment. The outputs of the comparatorscan be fanned in through a fast OR circuit which will produce a triggersignal if any comparator input exceeds the set threshold. The triggersignal can also be used with a suitable delay to control the T/Hswitches to apply hold signal at the peak of the pulse from the shaperamplifier.

The data acquisition speed of the readout chip will also be increasedusing the extra versatility introduced by the comparators. The designshown in FIG. 12 does not tell which strip has the information so allstrips are readout to find the strip that has the signal. A logiccircuit can be added to the design which detects the channel with thelargest signal from the comparator outputs, applies a track and holdsignal, and connects the strip with the signal to the analog output pin.At the same time it can encode the address of the strip that has theinformation and output it as the address of the strip with the signal.There could be an occasional signal on more than one strip. Multi-hitscan be detected and an output can be generated to warn of a multi-hitsignal. The trigger signals are generated for each readout chip. Theyhave to be externally processed for the hodoscope in coincidence withthe calorimeter to produce the single trigger signal to activate thedata acquisition system. For extremely high signal rates this may not bepossible. In such a case each wafer or front end readout chip can beseparately readout in parallel using independent data acquisitionelectronics and tagging each event time by using an accurate clock. Thecalorimeter crystals are also individually readout and event timestagged by the same clock. Since the calorimeter is running at muchslower speeds, individual readout modules are not necessary and can bereadout in groups.

The data readout can be carried out in parallel and can be storedon-board using individual module memory. This is the key to achieve fastdata throughput rates. The data can be asynchronously accessed by thehost computer, analyzed and displayed on screen in real time. Dataacquisition rates of 1 to 10 MHz per readout chip (or silicon wafer) areachievable.

A block diagram of the readout electronics system is shown in FIG. 13.The electronics has two similar sections for the hodoscope and thecalorimeter readout. A true event is a coincidence between the hodoscopeand the calorimeter. The two master trigger signals from the hodoscopeand the calorimeter are sent to a coincidence unit to create the Comptondouble scatter event trigger. The Compton double scatter trigger signalis only generated if there is a master trigger signal from both thehodoscope and the calorimeter. This is the arrangement which does notemploy the time tagged data readout method. Time tagged data acquisitionwill only be used if absolutely necessary.

The Compton double scatter event trigger activates data acquisition forboth the hodoscope and the calorimeter simultaneously. Either CAMAC orVME bus modules can carry out the data acquisition. The CAMAC system isthe most cost effective. Faster computer interface busses such asFastbus, VME or VXI bus can also be used. The custom designed dataacquisition modules for the hodoscope will produce the necessarymicrostrip readout chip control electronics, such as the T/H (if notgenerated internally in the readout chip), a clear signal to reset theshift registers, and the clock pulse to multiplex each strip to theanalog output.

The analog input channels from different hodoscope planes are read outsynchronously with the clock pulse output. The module converts the pulseheight information received from the analog output pin to a digitalnumber. In parallel with reading the hodoscope data, it also digitizesthe signal(s) from the calorimeter. Immediately after reading out thelast signal it clears the hodoscope to reset the readout chip so that itcan receive the next event. It is assumed that the analog output of eachreadout chip in each detector plane is fanned in to allow a singlesignal to be sent to the readout module. It is also possible to design amicrostrip readout chip that can internally connect the strip which hasthe maximum signal to the analog output and also produce the encodedaddress of the strip. In such a case the clock output will not benecessary and the silicon microstrip detectors can be readoutasynchronously at a much faster rate.

The custom made CAMAC modules are connected to the CAMAC cratecontrollers which are standard devices and available off the shelf. Thecontrollers connect the modules to the data acquisition computer.Depending on the data rate and readout overhead, single or separatecomputers can be used to read the hodoscope and the calorimeter. Thecomputer stores data on a hard disk, optical drive, or nonvolatile RAMdepending on the application. If the data acquisition overhead is nothigh then one of the computers can analyze the data in real time or aseparate computer can access the storage media asynchronously. Theresults of the data analysis are imaged onto the field-of-view through adisplay system in real time.

The data analysis techniques for nondestructive evaluation inspectionresemble closely those of medical Computer Assisted Tomography (CAT)imaging. This type of imaging is based on the Radon transform and backprojection techniques and is standard in the industry. New iterativetechniques such as Maximum Likelihood and Maximum Entropy methods canalso be applied to enhance the image quality as can the DLAD techniquedescribed above.

If a calorimeter is not used the direction and the energy of theincident photon has to be measured in the hodoscope. This can beachieved by increasing the total thickness. These measurements can bemade by two scatters where the second scatter is a photoelectricabsorption. If an incident photon makes 3 or more scatters (i.e., it isover determined), then the Compton scatter angle and the energy of theincident photon can be determined more than one independent way even ifthe photon does not deposit its full energy in the silicon converter andescape. Such multiple Compton scatters can also lead to a reduction inthe azimuthal ambiguity (i.e., event ring) because the Compton scatteredphoton will be polarized and the third interaction position is dependenton the scattered photon direction.

Monte Carlo simulation of a hodoscope only system has been carried outusing the same geometry as discussed above without a CdZnTe calorimeterand with 20 silicon strip detector planes. Out of 3×10⁶ 141 keV gammarays incident on the detector, 22 percent made a single scatter andescaped out, 10 percent of which were absorbed by the photoelectriceffect. 7.7 percent of the incident gamma rays made two scatters, 20percent of these depositing their full energy in the hodoscope. 4.2percent of the incident gamma rays produced 3 or more scatters. Most ofthese, in theory, may be used to determine the incident photon energyand scatter angle.

If a pure single line source is used then a high sensitivity imagingmode can be applied with some reduction in spatial resolution. In thismode the background discrimination cannot be applied for doublescatters. The requirement that the double scattered photon must depositall of its energy in the hodoscope reduces the number of useful eventsfor imaging by 80 percent. Since the energy of the incident photon isknown than the missing energy of the escaped photon can be added to thesecond scatter and the scatter angle can be determined. This methodimproves the signal somewhat but also increases the background. However,for a hodoscope only system it may increase the good data rate by asignificant factor.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

1. A method of processing a signal with an integrated circuit,comprising: receiving an input signal to an input of a channel;connecting an amplifier to said input to amplify said input signal to anamplified input signal; shaping said amplified input signal to a shapedinput signal; producing a peak of said shaped input signal; generating atrigger signal from said shaped input signal; outputting said peak;outputting said trigger signal.
 2. The method of claim 1, furthercomprising: connecting a shaping amplifier to said amplifier; connectinga comparator to said amplifier; and adjusting a threshold of saidcomparator.
 3. The method of claim 1, further comprising selecting ashaping time of said amplifier.
 4. The method of claim 1, wherein saidamplifier is self resetting.
 5. The method of claim 1, furthercomprising outputting an address of said channel.
 6. The method of claim1, further comprising storing information, wherein said informationincludes at least one of the group consisting of setup data, input data,address data and output data.
 7. The method of claim 1 furthercomprising detecting an event in coincidence with another event.
 8. Themethod of claim 7 further comprising recording an arrival time of saidevent.
 9. The method of claim 1 further comprising encoding an address.10. The method of claim 1 further comprising producing an image.
 11. Amulti-channel integrated circuit, wherein each channel comprises: aninput that receives an input signal; an amplifier connected to saidinput to amplify said input signal; a shaping amplifier connected to anoutput of said amplifier to shape said amplified signal; a circuitconnected to an output of said shaping amplifier to produce a peaksignal corresponding to a peak of said shaped signal; a comparatorconnected to said shaping amplifier to produce a trigger signal; anoutput to output said peak signal; and a trigger output to output saidtrigger signal.
 12. The integrated circuit of claim 11, furthercomprising: a fast shaping amplifier; and a comparator connected to saidfast shaping amplifier.
 13. The integrated circuit of claim 12, furthercomprising a charge sensitive amplifier.
 14. The integrated circuit ofclaim 13, further comprising a control that adjusts a threshold of saidcomparator.
 15. The integrated circuit of claim 11 further comprising aself resetting amplifier.
 16. The integrated circuit of claim 11 furthercomprising a time tagging circuit.
 17. The integrated circuit of 11further comprising a coincidence circuit.
 18. The integrated circuit ofclaim 11 further comprising a time tagged data acquisition system. 19.The integrated circuit of claim 11 further comprising an addressencoding system.
 20. The integrated circuit of claim 11 furthercomprising a Radon transform system.